Coagulated yarn and manufacturing method thereof, carbon fiber precursor fiber, and method of manufacturing carbon fiber

ABSTRACT

A coagulated yarn, which is used to manufacture carbon fiber, is a coagulated yarn the surface hole diameter of which is 30 nm or less and the degree of swelling is less than 100% or a coagulated yarn the surface hole diameter of which is 30 nm or less and the inner hole diameter is 30 nm or less. The carbon fiber precursor fiber and the carbon fiber are obtained using such a coagulated yarn.

TECHNICAL FIELD

This disclosure relates to a carbon fiber suitable for use in aircraft members, automobile members, and ship members as well as the applications of sport gear such as golf shafts and fishing rods and other general industrial applications.

BACKGROUND

Carbon fibers have a low specific weight simultaneously with a high specific strength and a high specific modulus and, accordingly, they have been widely used in the sports and aerospace applications as a reinforcement fiber for composite materials. Carbon fibers have recently extended their use to automobile applications, civil engineering and building applications, pressure vessels, windmill blades and the like, and there is a demand for further improvement in their performance.

Performance of the carbon fiber is known to greatly depend on the performance of the precursor fiber for carbon fiber. In particular, surface irregularities in the surface layer of the precursor fiber for carbon fiber induce loss of carbon fiber strength, and a dry-wet spinning method which is more likely to form a smooth surface has been proposed. However, a technology capable of further improving the strength is widely investigated.

For example, WO 2010/143680 proposes a technology wherein intrusion of an oil agent is suppressed by controlling the conditions of the dry-wet spinning step and stretching-in-the bath step by the use of a water-base coagulation bath for densification of the surface layer part.

Japanese Unexamined Patent Publication (Kokai) No. HEI 2-74607 proposes a technology wherein the size of the voids in the coagulated yarn is reduced by conducting the dry-wet spinning in a coagulation bath comprising a paraffin hydrocarbon.

Japanese Unexamined Patent Publication (Kokai) No. 2010-100970 proposes a technology having characteristic features in the coagulation step. More specifically, the technology proposed in the Japanese Unexamined Patent Publication (Kokai) No. 2010-100970 is wherein a polymer solution of low concentration is gelled in a low temperature coagulation bath of an alcohol and the speed in the stretching step is increased by a high draw ratio to improve productivity.

While the effect of improving the carbon fiber strength is realized by the use of the coagulated yarn produced by densifying the surface layer described in WO 2010/143680 and the coagulated yarn having reduced voids described in Japanese Unexamined Patent Publication (Kokai) No. HEI 2-74607, the effect has been insufficient.

WO 2010/143680 discloses that stretching the coagulated yarn having the degree of swelling of up to 160% under particular conditions is preferable, and stretching the yarn having a degree of swelling of 100 to 155% is disclosed in the Examples. However, we found that the increase in strength is insufficient when the degree of swelling is 100% or higher. Japanese Unexamined Patent Publication (Kokai) No. HEI 2-74607 discloses the technology where the degree of swelling had been reduced. However, we found that, when the proportion of the paraffin hydrocarbon is increased to reduce the degree of swelling, the carbon fiber had low evenness and the surface layer had an increased pore size, and the effect of improving the strength had been limited despite the decrease in the degree of swelling conceivably because of the slow coagulation rate. With regard to the technology of the Japanese Unexamined Patent Publication (Kokai) No. 2010-100970, that technology certainly had the effect of improving the productivity while it did not necessarily have the effect of improving the strength. This is probably because of the difficulty of achieving the densification necessary to realize the high strength in the coagulation step due to the low polymer concentration of the polymer solution.

It could therefore be helpful to provide a coagulated yarn for producing a carbon fiber having a high strength, a precursor fiber for carbon fiber, and a carbon fiber prepared by using these.

SUMMARY

We thus provide a coagulated yarn having a pore size of the surface layer of 30 nm or less and a degree of swelling of less than 100%, or a pore size of the surface layer of 30 nm or less and a pore size of the inner layer of 30 nm or less.

The coagulated yarn has a pore size of the surface layer of 30 nm or less and a degree of swelling of less than 100%, or a pore size of the surface layer of 30 nm or less and a pore size of the inner layer of 30 nm or less, and such a constitution enables production of a precursor fiber for carbon fiber capable of producing a carbon fiber having a high strength and a carbon fiber having a high strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an image of the surface layer of the coagulated yarn of Example 1 taken by TEM.

FIG. 2 is an image of the inner layer of the coagulated yarn of Example 1 taken by TEM.

FIG. 3 is an image of the surface layer of the coagulated yarn of Comparative Example 1 taken by TEM.

FIG. 4 is an image of the inner layer of the coagulated yarn of Comparative Example 1 taken by TEM.

DETAILED DESCRIPTION

A carbon fiber having a high strength is produced by controlling the pore size of the surface layer of a coagulated yarn to a small size and also controlling the degree of swelling to an extremely low level. In another example, a carbon fiber having a high strength is produced by controlling the pore size of the surface layer of a coagulated yarn to a small size and also controlling pore size of the inner layer to a small size.

The precursor fiber for carbon fiber is a precursor fiber that can be produced into carbon fiber. An exemplary such precursor fiber for carbon fiber is produced by stretching the coagulated yarn.

Coagulated Yarn Pore Size of the Surface Layer of the Coagulated Yarn

The size of the surface layer pores in the coagulated yarn is 30 nm or less. The strength of the yarn will be higher when this size is smaller, and the pore size of the surface layer is preferably up to 20 nm and more preferably up to 10 nm. When the pore size of the surface layer is up to 1 nm, desolvation in the washing with water will take time and, therefore, the lower limit is approximately 1 nm. In view of the balance between the carbon fiber strength and processability, the pore size of the surface layer is preferably 1 nm to 10 nm.

The surface layer corresponds to the area in the periphery (up to 500 nm from the exterior periphery to the interior) of the cross section in the direction of the fiber diameter. The pore size is the size of the pore formed by the fibrillar structure of the coagulated yarn and voids in the fibrillar structure.

Degree of Swelling of the Coagulated Yarn

The degree of swelling of the coagulated yarn may be less than 100%. When the pore size of the surface layer is within the range as described above, the strength will increase with the decrease in the degree of swelling and, therefore, the degree of swelling is preferably less than 90%, and more preferably less than 85%. When the degree of swelling is up to 3%, desolvation by washing with water will be retarded and, therefore, the lower limit is approximately 3%. The degree of swelling is more preferably 3% to 85% in view of the balance between the carbon fiber strength and processability.

Pore Size of the Inner Layer of the Coagulated Yarn

The pore size of the inner layer of the coagulated yarn may be 30 nm or less. When the pore size of the surface layer is within such a range, the strength will increase with the decrease in the pore size of the inner layer and, accordingly, the pore size of the inner layer is preferably up to 20 nm, and more preferably up to 10 nm. When the pore size of the inner layer is up to 1 nm, desolvation in the washing with water will take time and, therefore, the lower limit is approximately 1 nm. In view of the balance between the carbon fiber strength and processability, the pore size of the inner layer is preferably 1 nm to 10 nm.

The inner layer corresponds to the circular area (with the diameter of up to 500 nm) concentric with the cross section of the fiber in the direction of the fiber diameter. The pore size is the size of the pore formed by the fibrillar structure of the coagulated yarn and voids in the fibrillar structure.

Production Method of the Coagulated Yarn

The coagulated yarn can be produced, for example, by the process comprising the step of polymer coagulation using a coagulation bath prepared by mixing a non-solvent having a solubility parameter of −11 to +20 in relation to the solubility parameter of polymer to form the coagulated yarn and solvent of the polymer solution to form the coagulated yarn at a non-solvent to solvent ratio of 1:9 to 9:1.

Preferably, the polymer solution is ejected from the spinneret in the spinning step, introduced in the coagulation bath in the coagulation step for precipitation of the polymer to form the coagulated yarn, washed with water, stretched in the bath, applied with an oil agent in the oil agent-application step, and dried in the drying step to thereby produce the precursor fiber for carbon fiber. The coagulated yarn may also be produced by wet spinning or dry-wet spinning of the polymer solution. In such process, the pore size and the degree of swelling may be controlled by the conditions used in the coagulation of the polymer solution in the coagulation bath, namely, the conditions used to precipitate the polymer in the polymer solution from the solvent.

Spinning Step

The spinning method used may be either wet spinning or dry-wet spinning. However, the temperature of the coagulation bath is preferably set at a low temperature as will be described below. In consideration of the spinnability, the polymer solution should also be at a temperature capable of realizing certain flowability. Accordingly, a difference in the temperature is often set between the temperature of the coagulation bath and the temperature of the polymer solution. Therefore, use of the dry-wet spinning is preferable in view of the ease of setting temperature difference between the coagulation bath and the polymer (temperature of the polymer ejection spinneret).

The polymer is not particularly limited as long as it can be made into a carbon fiber. Examples include polyacrylonitrile, a copolymer containing polyacrylonitrile as its main component, and a mixture containing polyacrylonitrile as its main component. Unless otherwise noted, a copolymer containing polyacrylonitrile as its main component is called “polymer.”

The solvent for the polymer is not particularly limited as long as the polymer is soluble in the solvent, and examples include dimethyl sulfoxide, dimethylformamide, and dimethyl acetamide.

Although concentration of the polymer in the polymer solution is not particularly limited, the concentration is preferably at least 10% by weight since the degree of swelling is likely to be reduced with the increase in the polymer concentration. The upper limit is not particularly limited as long as the polymer is soluble in the solvent, and the upper limit is generally up to 30% by weight. Often, a higher polymer concentration is also preferable in view of reducing the pore size.

The polymer solution is more likely to be flowable when the temperature of the polymer solution ejected from the spinneret is higher. On the other hand, the precipitation in the coagulation bath is easier when the polymer solution is at a lower temperature. When the polymer precipitation in the coagulation bath is facilitated, the pore size will be smaller since the pore size will not be developed in the liquid-liquid phase separation process. Accordingly, the polymer solution preferably has a temperature of 15 to 95° C.

Coagulation Step

The coagulated yarn can be produced, for example, by the process comprising the step of polymer coagulation using a coagulation bath prepared by mixing a non-solvent having a solubility parameter of −11 to +20 in relation to the solubility parameter of polymer to form the coagulated yarn and a solvent of the polymer solution to form the coagulated yarn at a non-solvent to solvent ratio of 1:9 to 9:1. The solubility parameter is Hansen solubility parameter (MPa^(0.5)).

The polymer is less soluble when the difference between the solubility parameter of the non-solvent and the solubility parameter of the polymer is large. We discovered that the degree of swelling and the pore size of inner layer can be reduced by choosing a non-solvent having a solubility parameter near that of the polymer. The solubility parameter of the non-solvent is preferably −9 to +15, and more preferably −7 to +10 in relation to the solubility parameter of the polymer. When polyacrylonitrile is used for the polymer, the solubility parameter of the polyacrylonitrile is 27.4 and the preferable solubility parameter of the non-solvent is 16.4 to 47.4. Examples of such non-solvent include methanol, ethanol, propanol, butanol, glycerin, ethylene glycol, propylene glycol, butanediol, acetic acid, ethyl acetate, acetone, benzene, toluene, xylene, cyclohexane, methyl ethyl ketone, and chloroform. The non-solvent as used herein is those wherein the polymer is precipitated upon addition of the polymer solution under the environment of normal pressure and normal temperature. The solubility parameter used is the value described in or calculated by the method described in a handbook (see, for example, Hansen Solubility Parameters A User's Handbook Second Edition, CRC Press (2007)). When the polymer is a mixture, difference between the solubility parameter (δ) of the non-solvent and the solubility parameter of each polymer are compared, and a non-solvent having a solubility parameter which is −11 to +20 in relation to the solubility parameter of at least one polymer is used. When the non-solvent is a mixture, 3 parameters, namely, dispersion forces (δ_(d)), dipolar intermolecular force (δ_(p)), and hydrogen bond (δ_(h)) are calculated by the addition corresponding to the volume fraction of the mixture, and the square root of the total sum of the square of each of the resulting 3 parameters is used for the solubility parameter of the non-solvent.

For example, when the non-solvent is a two-component mixture comprising non-solvents A and B, δ_(d), δ_(p), and δ_(h) of the mixed non-solvent are

_(d)

_(p)

_(h)]=[(φA

_(dA) +φB

_(dB)),(φA

_(pA) +φB

_(pB)),(φA

_(hA) +φB

_(hB))]

wherein φ_(A) and φ_(B) are volume fraction of the mixture, and φ_(A)+φ_(B)=1. The solubility parameter (δ) of the mixed non-solvent can be calculated from the calculated δ_(d), δ_(p), and δ_(h) of the mixed non-solvent by:

5=√{square root over ((δd)²+(δp)²+(δh)²)}

We also discovered that the pore size of the surface layer and the inner layer can be controlled by mixing a solvent for the polymer in the coagulation bath. In addition, we discovered that while circularity tends to decrease when the non-solvent in the range as described above was used, an increase of the polymer solvent had the effect of increasing the circularity while retaining the low degree of swelling. On the other hand, we also found that the pore size of the surface layer and the inner layer can be reduced by reducing the amount of the polymer solvent. The ratio of the non-solvent to solvent may more preferably be 2:8 to 8:2, still more preferably 3:7 to 7:3, and most preferably 4:6 to 6:4. In addition, other substances may be present to the extent not adversely affecting the desired merits. The ratio as used herein is the ratio in weight.

The diffusion coefficient D of the non-solvent in the coagulation bath is preferably up to 3.4×10⁻¹⁰ m²·S⁻¹. When the diffusion coefficient D is low, the resulting coagulated yarn will exhibit a lower degree of swelling and the surface and inner layers will have a smaller pore size. The diffusion coefficient D of the non-solvent is the coefficient obtained by pulsed magnetic field gradient nuclear magnetic resonance (PFG-NMR method). In the PFG-NMR, diffusion movement distance of the substance, namely, information on the position of the nuclear spin can be collected by applying a pulsed magnetic field gradient (PFG) in the direction of the static magnetic field in the ordinary NMR analysis.

More specifically, this method is a method wherein attenuation of the observed peak strength based on the change of the PFG strength is tracked, and the diffusion coefficient is determined from the gradient obtained by exponential function analysis of the attenuation change. In the actual measurement of the non-solvent diffusion coefficient D using the PFG-NMR, an NMR system equipped with Diff60 probe (AVANCE III HD 400 manufactured by Bruker Biospin) is used, and the evaluation was conducted by Stejskal-Tanner equation:

ln(I/I ₀)=−Dγ ² G ²α²(Δ−α/3)

wherein G is intensity of magnetic field gradient, α is pulse width of magnetic field gradient, Δ is interval of the magnetic field gradient pulse (diffusion time), and γ is nuclear gyromagnetic ratio of the observing nucleus. ln(I/I₀) which is the signal strength I standardized with the signal strength I₀ when G is minimum is plotted in relation to the G²γ²α²(Δ−α/3), and the diffusion coefficient D of the non-solvent is determined from the gradient non-solvent. When two or more types of non-solvents are included, the D of the non-solvent having the highest diffusion coefficient D (the non-solvent with the highest diffusion speed) was defined as the D of the coagulating liquid.

The coagulation bath preferably has a viscosity of 2 to 1000 mPa·s. When the coagulation bath has a high viscosity, the degree of swelling is likely to be low, while low viscosity of the coagulation bath facilitates precipitation of the polymer and decrease of the pore size. The viscosity of the coagulation bath is more preferably 5 to 500 mPa·s, and still more preferably 10 to 200 mPa.

In the coagulation bath, the temperature of the coagulation bath is preferably 10 to 100° C. lower than the temperature of the polymer ejected from the spinneret. When the temperature of the coagulation bath is low, polymer precipitation is promoted, and the pore size can be controlled to a smaller size. On the other hand, when the coagulation bath has a high temperature, spinnability will be improved, and production of fibers with reduced fluffs and fiber-fiber adhesion will be enabled. The temperature of the coagulation bath is preferably 20 to 80° C., more preferably 30 to 60° C. lower than the temperature of the polymer solution.

Production Method of the Precursor Fiber for Carbon Fiber

Next, the method of producing the precursor fiber for carbon fiber is described.

The production method of the precursor fiber for carbon fiber preferably comprises forming the coagulated yarn by the method as described above and conducting the stretching step. More preferably, the precursor fiber for carbon fiber is produced by washing the coagulated yarn with water, stretching the washed yarn in the bath, applying an oil agent, and drying the yarn. In another preferable example, the method may further comprise a dry heat stretching step or a steam stretching step. The fiber after the coagulation may be directly subjected to the stretching in the bath without conducting the step of washing with water, or the fiber may be subjected to the stretching in the bath after removing the solvent by the washing with water.

After the stretching step in the bath, a silicone oil agent is preferably applied to the stretched fibers to prevent adhesion between the filaments.

After applying the oil agent, the fibers are preferably dried. In addition, after the drying step, the fibers are preferably stretched in a heated medium for the purpose of improving the productivity or orientation parameter of crystallites. Examples of the preferable heated medium include steam under pressure and superheated steam in view of the stable operation and the cost.

When the draw ratio is high, arrangement of the molecules in the fiber axis direction is promoted, and the tensile strength after production into the carbon fiber is more likely to be improved. On the other hand, when the draw ratio is reduced, improvement in the evenness in the fiber longitudinal direction is more likely to be becomes easier. Accordingly, the total draw ratio is preferably at least 1 and less than 20.

Production Method of the Carbon Fiber

Next, the method of producing the carbon fiber is described.

The production method of the carbon fiber preferably comprises the steps of producing the precursor fiber for carbon fiber and subjecting the precursor fiber for carbon fiber to a heat treatment. The heat treatment step is not particularly limited as long as the precursor fiber for carbon fiber is heated in the production of the carbon fiber from the precursor fiber for carbon fiber. Examples of such step include the stabilization (oxidation) or fireproofing step, pre-carbonization step, carbonization step, and graphitization step as described below.

Preferably, the precursor fiber for carbon fiber produced as described above is sequentially subjected to a stabilization step wherein the fibers are made fireproof in the air at a temperature of 200 to 300° C., a pre-carbonization step wherein the fibers produced by the stabilization step is preliminarily carbonized in an inert atmosphere at a temperature of 300 to 800° C., and a carbonization step wherein the fibers produced by the pre-carbonization step are carbonized in an inert atmosphere at a temperature of 1,000 to 3,000° C. to produce the carbon fiber.

When a carbon fiber having a higher modulus is desired, graphitization may be conducted subsequent to the carbonization step. The temperature of the graphitization step is preferably 2,000 to 2,800° C., and the maximum temperature may be adequately selected depending on the properties required for the intended carbon fiber. The draw ratio in the graphitization step may be adequately selected depending on the properties required for the intended carbon fiber so that the quality loss such as fluff generation will not be caused.

Surface Modification Step

The resulting carbon fiber may be subjected to electrolytic treatment for the surface modification so that the fiber reinforced composite material obtained by the electrolytic treatment will exhibit adequate adhesion between the carbon fiber and the carbon fiber matrix. This will solve the problems such as brittle fracture of the composite material by excessively high adhesion, loss of tensile strength in the fiber direction, and failure to develop the strength properties in the non-fiber direction due to high tensile strength in the fiber direction but with poor adhesion with the resin. As a consequence, the resulting fiber reinforced composite material will exhibit well-balanced strength properties in both fiber direction and non-fiber direction.

After the electrolytic treatment, sizing process may be conducted to provide convergency with the carbon fiber. The sizing agent may be an adequately selected sizing agent having good compatibility with the matrix resin depending on the type of the resin used.

EXAMPLES

The data in the Examples and the Comparative Examples were measured by the procedures as described below.

1. Pore Size of the Coagulated Yarn (1) Sample Preparation

The liquid in the coagulated yarn was replaced with water, and after freeze drying, this yarn was embedded in a resin. Sections of 100 nm were prepared with an ultramicrotome.

(2) Observation

After removing the resin in the prepared section, the section was observed by using a transmission electron microscope at an acceleration voltage of 100 kV. The cross-section in the fiber diameter direction was observed at a magnification of 10,000.

(3) Measurement of the Pore Size

A. Noise was removed by Image Processing Software J Trim ver. 1.53c (J Trim) at the applied intensity of 50. B. Normalization of the histogram for the image obtained in A was conducted with the J Trim. C. Black and white conversion was conducted by using 145 for the border threshold for the image obtained in B by the J Trim. D. In the image obtained in C, necessary areas were selected with an area selection tool of Image Processing Software Image J 1.50i (Image J) (the area with the width of 500 nm from the outer periphery was selected for the surface layer, and the circular area having a diameter of 500 nm wherein the center was the center of gravity of the cross-section was used for the inner layer). E. For the image obtained in D, the area of the part corresponding to the pore was measured by particle analyzing command of the Image Processing Software Image J 1.50i (Image J), and the thus obtained area was converted to a circle to determine the particle size. F. Of the particle size obtained in the image obtained in E, average of the second largest to the 31st largest was used for the particle size. When 31 particles were not detected, the values in the detected range were used.

2. Degree of swelling of the coagulated yarn

First, about 10 g of the coagulated yarn was sampled, and washed with water for at least 12 hours. Next, the yarn was dehydrated by using a centrifugal dehydrator (for example, H-110A manufactured by Kokusan) at 3000 rpm for 3 minutes and fiber weight after the dehydration was determined. Subsequently, the dehydrated sample was dried in a dryer adjusted to 105° C. for 2.5 hours, and the fiber weight after the drying was determined. The degree of swelling of the fiber was calculated by the following equation.

Degree of fiber swelling (%)=((fiber weight after the water removal−fiber weight after the drying)/fiber weight after the drying))×100

3. Tensile Strength and Modulus of the Carbon Fiber Bundle

The value was determined on the bases of JIS R7608 (2007) “Carbon fiber: Determination of tensile properties of resin-impregnated yarn”. The resin-impregnated strand of the carbon fiber to be measured was prepared by impregnating carbon fiber or graphitized fiber with 3,4-epoxycyclohexylmethyl-3,4-epoxy-cyclohexyl-carboxylate (100 parts by weight)/ boron trifluoride monoethylamine (3 parts by weight)/acetone (4 parts by weight) and curing at a temperature of 130° C. for 30 minutes. The number of carbon fiber strand measured was 6, and the average of the measurements was used for the tensile strength. In the Example, “Bakelite” (Registered Trademark) ERL4221 manufactured by Union Carbide was used for the 3,4-epoxycyclohexylmethyl-3 ,4-epoxy-cyclohexyl-carboxylate.

4. Measurement of the Diffusion Coefficient (D) of the Non-Solvent

The diffusion coefficient (D) of the non-solvent in each coagulation bath was measured by PFG-NMR method using NMR system (AVANCE III HD400 manufactured by Bruker Biospin) equipped with Diff60 probe. The measurement was conducted at a temperature of 5° C.

Example 1

A copolymer comprising acrylonitrile and itaconic acid was polymerized by solution polymerization using dimethyl sulfoxide for the solvent and using a polymerization initiator. The resulting polyacrylonitrile copolymer was used for the spinning dope solution.

The spinning dope solution was controlled to 50° C., and after ejecting into air, it was introduced in a coagulation bath prepared by mixing at 48% by weight of dimethyl sulfoxide (polymer solvent) and 52% by weight of ethylene glycol (non-solvent) controlled to 5° C., and the coagulated fiber was produced by dry-wet spinning by taking up at a speed corresponding to a spinning draft of 2.5. The coagulated fiber was washed with water and stretched in water bath. Subsequently, an amino-modified silicone-based silicone oil agent was applied to the fiber bundle that had been stretched in water bath, and the drying densification process was conducted by using a heater roller, and stretched in a steam at elevated pressure to a total draw ratio of the process of 10 to produce a polyacrylonitrile precursor fiber for carbon fiber having a filament fineness of 0.8 dtex. Next, the resulting polyacrylonitrile precursor fiber for carbon fiber was subjected to a stabilization process in air having a temperature gradient of 220 to 270° C. to produce a stabilized bundle of fibers. The resulting stabilized bundle of fibers was subjected to preliminary carbonization treatment in a nitrogen atmosphere at a temperature of 300 to 800° C. to produce a bundle of preliminarily carbonized fibers. The resulting bundle of preliminarily carbonized fibers was subjected to carbonization treatment in a nitrogen atmosphere at the maximum temperature of 1400° C. Subsequently, the fiber was subjected to electrolytic surface treatment by using aqueous solution of sulfuric acid for the electrolyte, washed with water, and dried, and carbon fiber was obtained after applying a sizing agent.

Example 2

The procedure of Example 1 was repeated except that methanol was used for the non-solvent in the coagulation bath to obtain the carbon fiber.

Example 3

The procedure of Example 1 was repeated except that temperature of the coagulation bath was controlled to 45° C.

Example 4

The procedure of Example 1 was repeated except that n-butanol was used for the non-solvent in the coagulation bath and ratio of the non-solvent to the polymer solvent was changed to obtain the carbon fiber.

Example 5

The procedure of Example 1 was repeated except that glycerin and ethanol were used for the non-solvent in the coagulation bath to obtain the carbon fiber.

Example 6

The procedure of Example 1 was repeated except that dimethylformamide was used for the polymer solvent to obtain the carbon fiber.

Example 7

The procedure of Example 1 was repeated except that ethylene glycol and ethanol were used for the non-solvent in the coagulation bath and ratio of the non-solvent to the polymer solvent was changed to obtain the carbon fiber.

Example 8

The procedure of Example 1 was repeated except that propylene glycol and ethanol were used for the non-solvent in the coagulation bath and ratio of the non-solvent to the polymer solvent was changed to obtain the carbon fiber.

Example 9

The procedure of Example 1 was repeated except that water and glycerin were used for the non-solvent in the coagulation bath and ratio of the non-solvent to the polymer solvent was changed to obtain the carbon fiber. The D evaluated by the PFG-NMR was 2.7×10⁻¹⁰ m²·S⁻¹.

Example 10

The procedure of Example 1 was repeated except that water and ethylene glycol were used for the non-solvent in the coagulation bath and ratio of the non-solvent to the polymer solvent was changed to obtain the carbon fiber. The D evaluated by the PFG-NMR was 2.7×10⁻¹⁰ m²·S⁻¹.

Example 11

The procedure of Example 10 was repeated except that temperature of the coagulation bath was controlled to 25° C.

Example 12

The procedure of Example 10 was repeated except that temperature of the coagulation bath was controlled to −15° C.

Example 13

The procedure of Example 1 was repeated except that water and propylene glycol were used for the non-solvent in the coagulation bath and ratio of the non-solvent to the polymer solvent was changed to obtain the carbon fiber. The D evaluated by the PFG-NMR was 3.3×10⁻¹⁰ m²·S⁻¹.

Example 14

The procedure of Example 1 was repeated except that water and methanol were used for the non-solvent in the coagulation bath and ratio of the non-solvent to the polymer solvent was changed to obtain the carbon fiber. The D evaluated by the PFG-NMR was 4.4×10⁻¹⁰ m²·S⁻¹.

Example 15

The procedure of Example 1 was repeated except that water and ethanol were used for the non-solvent in the coagulation bath and ratio of the non-solvent to the polymer solvent was changed to obtain the carbon fiber. The D evaluated by the PFG-NMR was 3.4×10⁻¹⁰ m²·S⁻¹.

Example 16

The procedure of Example 1 was repeated except that water and 1-propanol were used for the non-solvent in the coagulation bath and ratio of the non-solvent to the polymer solvent was changed to obtain the carbon fiber. The D evaluated by the PFG-NMR was 5.3×10⁻¹⁰ m²·S⁻¹.

Comparative Example 1

The procedure of Example 1 was repeated except that water was used for the non-solvent in the coagulation bath and ratio of the non-solvent to the polymer solvent was changed to obtain the carbon fiber. The D evaluated by the PFG-NMR was 3.5×10⁻¹⁰ m²·S⁻¹.

Comparative Example 2

The procedure of Example 1 was repeated except that no polymer solvent was used in the coagulation bath to obtain the carbon fiber.

Comparative Example 3

The procedure of Example 1 was repeated except that liquid paraffin and decanol were used for the non-solvent and no polymer solvent was used in the coagulation bath to obtain the carbon fiber. The type and the combination of the non-solvent used were the same as the examples described in Japanese Unexamined Patent Publication (Kokai) No. HEI 2-74607. The resulting coagulated yarn had a pore size of the surface layer of 42 nm.

Comparative Example 4

The procedure of Example 1 was repeated except that water was used for the non-solvent and dimethylformamide was used for the polymer solvent in the coagulation bath, and ratio of the non-solvent to the polymer solvent was changed to obtain the carbon fiber. The type of the non-solvent, the type of the solvent, and the mixing ratio used were the same as the examples described in WO 2010/143680. The resulting coagulated yarn had a pore size of the surface layer of 35 nm and a degree of swelling of 108%. The D evaluated by the PFG-NMR was 5.5×10⁻¹⁰ m²·S⁻¹.

Comparative Example 5

The procedure of Example 1 was repeated except that water was used for the non-solvent in the coagulation bath and ratio of the non-solvent to the polymer solvent was changed to obtain the carbon fiber. The D evaluated by the PFG-NMR was 5.8×10⁻¹⁰ m²·S⁻¹.

TABLE 1 Production conditions Physical properties Mixing Temper- ratio ature (Weight difference fraction) between Non- Non- the Coagulated yarn solvent solvent Viscos- polymer Pore Pore (total) (1)/ ity solution size size Solu- non- of the and the of of Non-solvent (1) Non-solvent (2) bility solvent coagu- coagu- the the Degree Carbon Solubility Solubility para- Solvent (2)/ lation lation surface inner of fiber parameter parameter meter — solvent bath bath layer layer swelling Strength — (MPa)^(0.5) — (MPa)^(0.5) (MPa)^(0.5) — — mPa · s ° C. nm nm % GPa Example Ethylene 33 33 DMSO 52/0/48 17 45 3 3 82 8.9 1 glycol Example Methanol 29.6 29.6 DMSO 52/0/48 10 45 12 15 85 8.5 2 Example Ethylene 33 33 DMSO 52/0/48 17 5 15 3 84 8.5 3 glycol Example n-butanol 23.2 23.2 DMSO 40/0/60 15 45 7 10 83 8.7 4 Example Glycerine 36.2 Ethanol 26.5 30.2 DMSO  26/26/48 50 45 4 18 82 8.7 5 Example Ethylene 33 33 DMF 52/0/48 15 45 5 5 80 8.7 6 glycol Example Ethylene 33 Ethanol 26.5 27.7 DMSO  20/60/20 16 45 9 7 85 8.7 7 glycol Example Propylene 30.2 Ethanol 26.5 27.3 DMSO  20/60/20 14 45 12 7 87 8.5 8 glycol Example Water 47.8 Glycerine 36.2 39.5 DMSO  20/60/20 20 45 8 5 85 8.7 9 Example Water 47.8 Ethylene 33 36.8 DMSO  20/60/20 18 45 19 10 88 8.5 10 glycol Example Water 47.8 Ethylene 33 36.8 DMSO  20/60/20 18 25 21 12 90 8.3 11 glycol Example Water 47.8 Ethylene 33 36.8 DMSO  20/60/20 18 65 17 8 84 8.3 12 glycol Example Water 47.8 Propylene 30.2 33.9 DMSO  20/60/20 14 45 19 12 90 8.4 13 glycol Example Water 47.8 Methanol 29.6 33.2 DMSO  20/60/20 7 45 28 19 96 8.2 14 Example Water 47.8 Ethanol 26.5 30.6 DMSO  20/60/20 9 45 26 21 92 8.3 15 Example Water 47.8 1- 24.6 29.0 DMSO  20/60/20 7 45 22 20 98 8.1 16 Propanol Compar- Water 47.8 47.8 DMSO 20/0/80 8 45 32 50 102 8 ative Example 1 Compar- Ethylene 33 33 (DMSO) 100/0/0   16 45 40 3 85 7.5 ative glycol Example 2 Compar- Liquid 16.2 1-decanol 19.4 16.4 (DMSO) 90/10/0 70 45 42 3 80 7.7 ative paraffin Example 3 Compar- Water 47.8 47.8 DMF 20/0/80 7 45 35 42 108 7.7 ative Example 4 Compar- Water 47.8 47.8 DMSO 50/0/50 7 45 48 62 131 7.1 ative Example 5 

1-11. (canceled)
 12. A coagulated yarn used to produce a precursor fiber for carbon fiber, wherein pore size of the surface layer is 30 nm or less and degree of swelling is less than 100%.
 13. A coagulated yarn used to produce a precursor fiber for carbon fiber, wherein pore size of the surface layer is 30 nm or less and pore size of the inner layer is 30 nm or less.
 14. The coagulated yarn according to claim 12, wherein the pore size of the surface layer is 30 nm or less and pore size of the inner layer is 30 nm or less.
 15. A method of producing the coagulated yarn according to claim 12 comprising coagulating the polymer using a coagulation bath prepared by mixing a non-solvent having a solubility parameter of −11 to +20 in relation to the solubility parameter of the polymer to form the coagulated yarn and a solvent of the polymer to form the coagulated yarn at a non-solvent to solvent ratio of 1:9 to 9:1.
 16. The method according to claim 15 using a coagulation bath, wherein the non-solvent has a diffusion coefficient of up to 3.4×10⁻¹⁰ m²·S−1.
 17. The method according to claim 15, wherein the coagulation bath has a viscosity of 2 to 1000 mPa·s.
 18. The method according to claim 15, wherein the coagulation bath has a temperature 10 to 100° C. lower than the temperature of the polymer solution.
 19. The method according to claim 15, further comprising spinning at a spinning draft of 1 to
 20. 20. A method of producing a precursor fiber comprising stretching the coagulated yarn according to claim
 12. 21. A method of producing precursor fiber for carbon fiber comprising obtaining a coagulated yarn by the method according to claim 15 and stretching the coagulated yarn.
 22. A method of producing a carbon fiber comprising producing a precursor fiber for carbon fiber by the method according to claim 20 and subjecting the precursor fiber for carbon fiber to a heat treatment.
 23. A method of producing the coagulated yarn according to claim 13 comprising coagulating the polymer using a coagulation bath prepared by mixing a non-solvent having a solubility parameter of −11 to +20 in relation to the solubility parameter of the polymer to form the coagulated yarn and a solvent of the polymer to form the coagulated yarn at a non-solvent to solvent ratio of 1:9 to 9:1.
 24. A method of producing the coagulated yarn according to claim 14 comprising coagulating the polymer using a coagulation bath prepared by mixing a non-solvent having a solubility parameter of −11 to +20 in relation to the solubility parameter of the polymer to form the coagulated yarn and a solvent of the polymer to form the coagulated yarn at a non-solvent to solvent ratio of 1:9 to 9:1.
 25. The method according to claim 16, wherein the coagulation bath has a viscosity of 2 to 1000 mPa·s.
 26. The method according to claim 16, wherein the coagulation bath has a temperature 10 to 100° C. lower than the temperature of the polymer solution.
 27. The method according to claim 17, wherein the coagulation bath has a temperature 10 to 100° C. lower than the temperature of the polymer solution.
 28. The method according to claim 16, further comprising spinning at a spinning draft of 1 to
 20. 29. The method according to claim 17, further comprising spinning at a spinning draft of 1 to
 20. 30. The method according to claim 18, further comprising spinning at a spinning draft of 1 to
 20. 